INR Calculator
Your measured PT in seconds from the lab report. Must be greater than zero.
Your laboratory's mean normal PT in seconds (also called control PT or mean normal). Typically 9.8–12.7 seconds. Check your lab report or ask your lab.
The ISI value for your laboratory's thromboplastin reagent. Typically 0.9–1.7 for modern reagents. Often printed on your lab report or available from your lab.
Select your condition to check if your INR is within the recommended therapeutic range for that indication.
Enter Your Lab Values
Enter your patient PT, normal PT, and ISI from your lab report to calculate your INR and see clinical interpretation.
How to Use the INR Calculator
Locate Your Lab Report Values
Find your prothrombin time (PT) result on your lab report — it will be listed in seconds. You also need the normal PT (sometimes called control PT or mean normal PT), which is the reference value for healthy patients at your specific laboratory. Finally, locate the ISI (International Sensitivity Index) for the reagent used — this is usually printed on your lab report or available from your lab, and is typically between 0.9 and 1.7 for modern reagents.
Enter Values and Select Your Indication
Enter your patient PT, normal PT, and ISI into the three input fields. Optionally, select your clinical indication from the dropdown (e.g., atrial fibrillation, DVT, mechanical heart valve) to have the calculator automatically check whether your INR falls within the recommended therapeutic range for your specific condition. The INR calculates automatically as you type.
Review Your INR, Status, and Clinical Guidance
The results panel shows your calculated INR value, PT ratio, and interpretation status (Normal, Subtherapeutic, Therapeutic, Supratherapeutic, Critical, etc.). The INR zone gauge gives a visual overview of where you fall on the spectrum. The clinical guidance section shows a recommended dose action, bleeding risk level, and suggested recheck timing based on evidence-based anticoagulation guidelines.
Use the Advanced Tools
Expand the Warfarin Dose Adjustment Calculator by entering your current weekly dose to get a suggested new weekly dose. Use the INR Trend Tracker to enter previous readings and see whether your INR is trending up, down, or stable. Click the Factors panel to review which medications and foods may be affecting your INR. Export your result to CSV or print it for your medical appointment.
Frequently Asked Questions
What is a normal INR value?
For a healthy adult not taking any anticoagulation medication, a normal INR is 0.8 to 1.2. This corresponds to a normal prothrombin time of approximately 9.8 to 12.7 seconds in most laboratories. An INR below 0.8 is unusually low and may suggest a hypercoagulable state, though this is rarely clinically significant in otherwise healthy individuals. An INR above 1.2 in a non-anticoagulated patient warrants further investigation for potential coagulation factor deficiencies, liver disease, vitamin K deficiency, or other conditions affecting the clotting cascade. If you are taking warfarin, your target INR will be higher than normal — typically 2.0 to 3.0 — and an INR of 1.0 to 1.5 would actually be considered subtherapeutic for your condition.
What is the target INR for warfarin therapy?
The target INR for most warfarin indications is 2.0 to 3.0. This range applies to atrial fibrillation, deep vein thrombosis, pulmonary embolism, tissue (bioprosthetic) heart valves (usually only for the first 3 months after surgery), valvular heart disease, and antiphospholipid syndrome. Patients with mechanical prosthetic heart valves generally require a higher target of 2.5 to 3.5 because the risk of clot formation on metal valve surfaces is greater. An exception is the On-X mechanical aortic valve: after an initial therapeutic period of 3 months combined with daily aspirin, guidelines allow a reduced target of 1.5 to 2.0. Catheter-related venous thrombosis prevention uses a low-intensity target of 1.2 to 1.5. Your physician will specify your individual target based on your condition and risk factors.
My INR is above 3.0 — should I be worried?
Whether an elevated INR is concerning depends on your target range and how high the value is. If your target is 2.0 to 3.0 and your INR is 3.1 to 3.9, this is mildly supratherapeutic — it may require a small dose reduction but is not typically dangerous without active bleeding. An INR of 4.0 to 5.0 represents significant supratherapeutic anticoagulation and usually requires holding one or two warfarin doses and reducing your weekly dose, along with same-day contact with your healthcare provider. An INR above 5.0 is considered critical and requires immediate provider contact, holding warfarin, and possible vitamin K administration. An INR above 9.0 is a medical emergency requiring urgent evaluation. If you are experiencing any bleeding symptoms — including unusual bruising, nosebleeds, blood in urine or stool, or severe headache — seek emergency care immediately regardless of your exact INR value.
Why does my INR fluctuate?
INR fluctuation is extremely common in warfarin patients and reflects the drug's sensitive interaction with many physiological variables. The most common causes include changes in vitamin K intake — eating more or fewer leafy green vegetables than usual can shift your INR significantly in either direction. Medications are another major source of variation: antibiotics (especially fluoroquinolones, macrolides, and metronidazole), antifungals, amiodarone, and NSAIDs tend to increase INR, while rifampin, some antiepileptics, and St. John's Wort decrease it. Illness, fever, diarrhea, alcohol use, and changes in liver function all affect how your body processes warfarin and vitamin K. Even small changes in diet consistency, sleep, or hydration can have measurable effects. This is why consistent habits — rather than dietary avoidance — are the foundation of stable anticoagulation management.
What does the ISI value mean and where do I find it?
The International Sensitivity Index (ISI) is a calibration number assigned by the manufacturer of the thromboplastin reagent used in your laboratory's PT test. It reflects how sensitive that particular reagent is to changes in clotting factor levels compared to the WHO International Reference Preparation. Modern recombinant thromboplastins typically have ISI values between 0.94 and 1.4 — the closer to 1.0, the more sensitive and consistent the reagent. Older reagents had ISI values up to 3.0, which produced much more variable INR results across different labs. You can usually find the ISI on your laboratory report alongside the PT result, or by calling your laboratory directly. If you cannot find it, a default value of 1.0 to 1.3 is reasonable for most modern laboratories, but confirming the exact value gives the most accurate INR calculation.
Can I use this INR calculator for liver disease or DOACs?
This calculator is specifically designed for monitoring vitamin K antagonist (VKA) therapy — primarily warfarin. It is not appropriate for interpreting PT/INR in liver disease. The INR formula was calibrated using plasmas from patients on oral anticoagulants, not liver disease patients, and liver disease affects both pro-coagulant and anti-coagulant factors simultaneously in ways the INR formula does not capture. In liver disease, INR overestimates bleeding risk and does not accurately reflect overall hemostatic competence. Direct oral anticoagulants (DOACs) such as rivaroxaban (Xarelto), apixaban (Eliquis), edoxaban (Savaysa), and dabigatran (Pradaxa) do not require INR monitoring at all — they are given at fixed doses and do not affect the INR in a therapeutically meaningful way. If you are on a DOAC, this calculator does not apply to your therapy.